Water moving through a pipeline has significant force that tends to separate the joints at changes in flow direction (bends and tees), stops (plugs, caps, or closed valves), and changes in size (reducers). Something must be done to keep the joints from separating and to maintain the equilibrium of the pipeline. Three of the primary ways to do so are:
Mechanically – megalug – used on both plastic and ductile iron pipe – done at all ductile iron fitting locations.
Metal tie rods – used primarily on plastic pipe in special situations, primarily when a long length of pipe is installed after a major change in pipe direction. Tie rods are made up of a prefabricated metal harness on both sides of the joint being held together, which can make installation somewhat labor intensive. A tie-rod restraint has to be somehow protected against weakening by corrosion, so the equilibrium of the pipe is not jeopardized. This may be done with specially coated bolts, stainless steel bolts, or coating the tie-rod system with a tar or epoxy product.
Field Locking Gaskets – Used on ductile iron pipe in similar situations to tie-rods above.
Intermittent hydrostatic pressure thrusts can occur with water mains. Thrust blocks are used in vulnerable locations (tees, bends, caps, etc) to resist joints pulling out. They are often used in addition to other means of joint restraint and are commonly poured concrete (formed or loose) but they may be precast blocks as well. They must be sufficient in resisting the hydrostatic force based on the pipeline characteristics, type of soil, and bearing strength. Thrust blocks should be placed on undisturbed soil as forces in the pipeline are passed through the thrust block to the soil. Thrust blocks may be hand formed or earth formed.
Depending on pipe diameter, designed internal pressure, depth of cover, type of soil, trench construction type, fitting type, an engineer will determine how many feet a joint must be restrained in either direction to appropriately resist forces over a set period of time.